The present invention relates to a high strength drive shaft and a process for producing the same. The term xe2x80x9cstrength propertiesxe2x80x9d used herein refers mainly to torsional strength and torsional fatigue strength.
Regarding drive shafts, one of automobile components, in order to cope with an increase in power of automobile engines or atmospheric control in recent years, there is an ever-increasing trend toward an increase in strength of the drive shafts. Strength properties required of drive shafts are torsional strength and torsional fatigue strength.
Japanese Patent Publication No. 62571/1988 discloses a process for producing a drive shaft, comprising the steps of: forming a steel comprising carbon: 0.30 to 0.38%, manganese: 0.6 to 1.5%, boron: 0.0005 to 0.0030%, titanium: 0.01 to 0.04%, and aluminum: 0.01 to 0.04% into a drive shaft; and induction hardening the drive shaft to a ratio of the induction hardening depth to the radius of the steel member of not less than 0.4, thus enabling the step of tempering after the induction hardening to be omitted. This publication, however, does not refer to torsional fatigue strength to which the present invention has made particular attention as strength properties.
Japanese Patent Laid-Open No. 179400/1993 discloses a steel product for direct machining and induction hardening, comprising carbon: 0.38 to 0.45%, silicon: not more than 0.35%, manganese: more than 1.0 to 1.5%, boron: 0.0005 to 0.0035%, titanium: 0.01 to 0.05%, aluminum: 0.01 to 0.06%, and nitrogen: not more than 0.01%, the steel product having a fine grain structure with a ferrite grain size number of not less than 6. This laid-open publication also refers to torsional strength, but is silent on torsional fatigue strength. Further, this steel product is considered to have unsatisfactory cold workability due to high manganese, nitrogen, or silicon content.
The steel products disclosed in the publications noted above cannot provide, as steels for drive shafts, excellent strength properties, especially excellent torsional fatigue strength properties, without difficulty. Accordingly, it is an object of the present invention to solve the above problems of the prior art and to provide a high strength drive shaft and a process for producing the same.
The above object can be attained by the following invention. Specifically, according to one aspect of the present invention, there is provided a high strength drive shaft including an involute serration, said drive shaft comprising as a steel material by weight carbon: 0.48 to 0.58%, silicon: 0.01 to 0.15%, manganese: 0.35 to 0.75%, sulfur: 0.005 to 0.15%, molybdenum: 0.1 to 0.35%, boron: 0.0005 to 0.005%, aluminum: 0.015 to 0.05%, and titanium: 0.02 to 0.08% and, in addition, nitrogen: limited to not more than 0.005% (inclusive of 0%), chromium: limited to not more than 0.1% (inclusive of 0%), phosphorus: limited to not more than 0.02% (inclusive of 0%), and oxygen: limited to not more than 0.002% (inclusive of 0%), with the balance consisting of iron and unavoidable impurities, the proportion of the effective case depth of the involute serration in its end being 0.3 to 0.7 in terms of the ratio of the effective case depth t to the radius r, t/r. In this case, the involute serration in its end preferably has a former austenite grain size of not less than 8 in terms of the grain size number specified in JIS (Japanese Industrial Standards).
The above object can be attained by the following invention. Specifically, according to one aspect of the present invention, there is provided a high strength drive shaft including an involute serration, said drive shaft comprising as a steel material by weight carbon: 0.48 to 0.58%, silicon: 0.01 to 0.15%, manganese: 0.35 to 0.75%, sulfur: 0.005 to 0.15%, molybdenum: 0.1 to 0.35%, boron: 0.0005 to 0.005%, aluminum: 0.015 to 0.05%, and titanium: 0.02 to 0.08% and, in addition, nitrogen: limited to not more than 0.005% (inclusive of 0%), chromium: limited to not more than 0.1% (inclusive of 0%), phosphorus: limited to not more than 0.02% (inclusive of 0%), and oxygen: limited to not more than 0.002% (inclusive of 0%), with the balance consisting of iron and unavoidable impurities, the proportion of the effective case depth of the involute serration in its end being 0.3 to 0.7 in terms of the ratio of the effective case depth t to the radius r, t/r. In this case, the involute serration in its end preferably has a former austenite grain size of not less than 8 in terms of the grain size number specified in JIS (Japanese Industrial Standards). The grain size numbers of JIS and ASTM are the same in both standards.
Use of the high strength drive shaft and the process for producing the same according to the present invention can provide drive shaft products possessing excellent torsional strength and torsional fatigue strength properties.